In many cases, MRI apparatuses have been utilized to inspect cephalic regions, for example. It has been also expected that PET apparatuses are utilized to inspect cephalic regions, e.g., to diagnose Alzheimer diseases. Hence, recently, it has been expected to realize a PET-MRI apparatus combining a PET apparatus and an MRI apparatus.
The MRI apparatuses, however, have some limitations. For example, a photomultiplier tube (PMT) used as a detector of a conventional PET apparatus cannot be used when the PET-MRI apparatus is intended to be realized because a strong radio frequency magnetic field is used in the MRI apparatus. Therefore, a PET-MRI apparatus has been proposed that uses an APD (Avalanche Photodiode) or a SiPM (Silicon Photomultiplier) instead of the PMT, for example.
In some conventional PET-MRI apparatuses, a PET detector is disposed at the magnetic field center included in an effective imaging region of MR images. Such a disposition is the most suitable for taking PET images. However, in order to take MR images, this disposition is not always the most suitable because MR images having sufficient image quality are not obtained in some cases due to influence of material of the PET detector disposed around the magnetic field center.